1. Field of the Invention
The present invention relates to longitudinally-coupled-resonator-type surface acoustic wave filters and in particular relates to longitudinally-coupled-resonator-type surface acoustic wave filters in which conversion between an unbalanced signal and balanced signals is performed.
2. Description of the Related Art
In recent years, with the technological progress in mobile communication devices such as cellular phones, there has been a marked increase in the amount of data being communicated. Consequently, frequency bands are being preferentially assigned to mobile communication devices. In addition, in order to increase the speed of data communication, multiband technology in which a plurality of frequency bands are utilized is progressing.
The number of frequency filters used in a mobile communication device has been increasing in order to handle the various frequency bands. Consequently, the demand for small-sized frequency filters has been increasing and surface acoustic wave filters characterized by having a small size are widely used. Along with further reductions in size, there is also a demand for surface acoustic wave filters to have improved attenuation characteristics.
In order to improve attenuation characteristics, a longitudinally-coupled-resonator-type surface acoustic wave filter is generally used as disclosed in Japanese Unexamined Patent Application Publication No. 2006/527516. In a longitudinally-coupled-resonator-type surface acoustic wave filter, two surface acoustic wave filters are cascade connected in a direction orthogonal to a propagation direction of the surface acoustic waves.
FIG. 4 is a circuit diagram that schematically illustrates the electrode configuration and connections of a longitudinally-coupled-resonator-type surface acoustic wave filter of the related art, which is a balanced-balanced filter.
A longitudinally-coupled-resonator-type surface acoustic wave filter 4 illustrated in FIG. 4 is a balanced-balanced filter that transmits balanced signals. The longitudinally-coupled-resonator-type surface acoustic wave filter 4 includes a pair of balanced terminals 20 and a pair of balanced terminals 21. The longitudinally-coupled-resonator-type surface acoustic wave filter 4 is formed on a main surface of a piezoelectric substrate (not illustrated). The longitudinally-coupled-resonator-type surface acoustic wave filter 4 further includes a first surface acoustic wave filter unit 41, which is a first stage of a cascade connection, and a second surface acoustic wave filter unit 42, which is a second stage of the cascade connection. The first surface acoustic wave filter unit 41 and the second surface acoustic wave filter unit 42 are cascade connected with each other, whereby attenuation outside of the pass band can be increased.
The first surface acoustic wave filter unit 41 includes, on the piezoelectric substrate, a first central interdigital transducer (IDT) 411 that is arranged on a central axis x that is orthogonal to the propagation direction of surface acoustic waves, a pair of first both sides IDTs 412 that are arranged on the left and right sides of the first central IDT 411, and first reflectors 413 that are arranged such that the first both sides IDTs 412 are interposed therebetween.
The first central IDT 411 includes a pair of first central divided comb tooth shaped electrodes 411a obtained by dividing a comb tooth shaped electrode on the side close to the balanced terminals 21 along the central axis x in order to allow connection to the pair of balanced terminals 21. In addition, the first central IDT 411 includes a first central floating comb tooth shaped electrode 411b on the side spaced from the balanced terminals 21.
Similarly, the second surface acoustic wave filter unit 42 includes, on the piezoelectric substrate, a second central IDT 421 that is arranged on the central axis x so as to oppose the first central IDT 411, a pair of second both sides IDTs 422 that are arranged on the left and right sides of the second central IDT 421, and second reflectors 423 that are arranged so that the second both sides IDTs 422 are interposed therebetween.
The second central IDT 421 also includes a pair of second central divided comb tooth shaped electrodes 421a obtained by dividing a comb tooth shaped electrode on the side close to the balanced terminals 20 along the central axis x in order to allow connection to the pair of balanced terminals 20. In addition, the second central IDT 421 includes a second central floating comb tooth shaped electrode 421b on the side spaced from the balanced terminals 20.
Comb tooth shaped electrodes of the first both sides IDTs 412 and comb tooth shaped electrodes of the second both sides IDTs 422 that oppose each other along the central axis x are respectively connected to each other on the left and right sides.
In a balanced-balanced filter, balanced signals having opposite phases to each other (differential signals) are transmitted to a pair of balanced terminals. Consequently, there is no need for the first central IDT 411, which is connected to the balanced terminals 21, and the second central IDT 421, which is connected to the balanced terminals 20, to be connected to a reference potential at a comb tooth shaped electrode on the side spaced from the balanced terminals. Therefore, ground wiring lines for connecting the first central floating comb tooth shaped electrode 411b and the second floating comb tooth shaped electrode 421b to a ground potential (GND), which serves as a reference potential, can be omitted. Therefore, the longitudinally-coupled-resonator-type surface acoustic wave filter 4, which is a balanced-balanced filter, has a smaller area occupied by wiring lines and therefore is suitable for achieving size reduction.
However, the demand for unbalanced-balanced filters as frequency filters used in mobile communication devices is greater than that for balanced-balanced filters. In an unbalanced-balanced filter, one terminal is an unbalanced terminal and the other terminals are balanced terminals. An unbalanced-balanced filter is a frequency filter that performs conversion between an unbalanced signal and balanced signals. A longitudinally-coupled-resonator-type surface acoustic wave filter of the related art that is an unbalanced-balanced filter is disclosed in WO 2005/031971.
FIG. 5 is a circuit diagram that schematically illustrates the electrode configuration and connections of the longitudinally-coupled-resonator-type surface acoustic wave filter of the related art that is an unbalanced-balanced filter.
A longitudinally-coupled-resonator-type surface acoustic wave filter 5 illustrated in FIG. 5 is different from the longitudinally-coupled-resonator-type surface acoustic wave filter 4 illustrated in FIG. 4 in that it is equipped with a single unbalanced terminal 10 instead of the pair of balanced terminals 21.
A first surface acoustic wave filter unit 51 includes, on a piezoelectric substrate, a first central IDT 511 that is arranged on a central axis x that is orthogonal to a propagation direction of surface acoustic waves, a pair of first both sides IDTs 512 that are arranged on the left and right sides of the first central IDT 511, and first reflectors 513 that are arranged so that the first both sides IDTs 512 are interposed therebetween.
Comb tooth shaped electrodes of the first central IDT 511 are different from those of the first central IDT 411 illustrated in FIG. 4 and are not divided.
Similarly, a second surface acoustic wave filter unit 52 includes, on a piezoelectric substrate, a second central IDT 521 that is arranged on the central axis x so as to oppose the first central IDT 511, a pair of second both sides IDTs 522 that are arranged on the left and right sides of the second central IDT 521, and second reflectors 523 that are arranged so that the second both sides IDTs 522 are interposed therebetween.
The second central IDT 521 includes a pair of second central divided comb tooth shaped electrodes 521a obtained by dividing a comb tooth shaped electrode on the side close to the balanced terminals 20 along the central axis x. In addition, a second central floating comb tooth shaped electrode 521b on the side spaced from the balanced terminals 20 is not connected to the ground potential and is at a floating potential.
Comb tooth shaped electrodes of the first both sides IDTs 512 and comb tooth shaped electrodes of the second both sides IDTs 522 that oppose each other along the central axis x are respectively connected to each other on the left and right sides.
An unbalanced signal is transmitted to the unbalanced terminal. Consequently, the comb tooth shaped electrode on the side spaced from the unbalanced terminal has to be connected to a reference potential. Therefore, in the first central IDT 511, which is connected to the unbalanced terminal 10, a comb tooth shaped electrode on the side spaced from the unbalanced terminal 10 is connected to the ground potential.
At this time, a ground wiring line is formed on the inner side between a pair of interstage wiring lines that respectively connect the first surface acoustic wave filter unit 51 and the second surface acoustic wave filter unit 52 to each other on the left and right sides. Accordingly, the interstage wiring lines and the ground wiring line cross each other. In such a case, the interstage wiring lines and the ground wiring line are made to cross over each other three dimensionally as disclosed in WO 2006/009021.
FIG. 6 is a plan view illustrating the layout of IDTs and wiring lines formed on a main surface of a piezoelectric substrate 500 in the longitudinally-coupled-resonator-type surface acoustic wave filter 5 illustrated in FIG. 5. In FIG. 6, a wiring layer in which terminal pad electrodes and wiring lines are formed is schematically illustrated. Positions of the IDTs are illustrated with broken lines and the same symbols as in FIG. 5 are used.
In the first surface acoustic wave filter unit 51 (not illustrated), the first central IDT 511 is arranged on the central axis x. The pair of first both sides IDTs 512 are arranged on the left and right sides of the first central IDT 511.
In the second surface acoustic wave filter unit 52 (not illustrated), the second central IDT 521 being divided along the central axis x is arranged. The pair of second both sides IDTs 522 are arranged on the left and right sides of the second central IDT 521. In addition, a floating electrode pad 50e is formed on the side of the second central IDT 521 that is spaced from balanced terminal electrode pads 20e. The floating electrode pad 50e is not connected to the ground potential and is at a floating potential. The second central floating comb tooth shaped electrode 521b is connected to the floating electrode pad 50e. 
An unbalanced signal wiring line 10f is formed so as to connect a single unbalanced terminal electrode pad 10e and the first central IDT 511 on the side close to the unbalanced terminal electrode pad 10e to each other.
Balanced signal wiring lines 20f are formed so as to connect the pair of balanced terminal electrode pads 20e and the second central IDT 521 on the side close to the balanced terminal electrode pads 20e to each other on the left and right sides.
Interstage wiring lines 30f are formed so as to connect the first both sides IDTs 512 and the second both sides IDTs 522, which oppose each other, to each other on the left and right sides.
Ground wiring lines 40f are formed in regions outside of the IDTs so as to connect ground terminal electrode pads 40e, the first both sides IDTs 512, which are on the side close to the unbalanced terminal electrode pad 10e and the second both sides IDTs 522, which are on the side close to the balanced terminal electrode pads 20e, to one another on the left and right sides.
In addition, in an interstage region between the first surface acoustic wave filter unit 51 and the second surface acoustic wave filter unit 52, a ground wiring line 41f is formed so as to connect the ground terminal electrode pads 40e and the first central IDT 511 on the side spaced from the unbalanced terminal electrode pad 10e to each other on the left and right sides.
In the longitudinally-coupled-resonator-type surface acoustic wave filter 5, the interstage wiring lines 30f and the ground wiring line 41f cross each other. Consequently, multilevel crossing portions 60 are provided in which the interstage wiring lines 30f and the ground wiring line 41f cross each other three dimensionally. In the multilevel crossing portions 60, insulating layers (not illustrated) are formed between an upper layer and a lower layer so that the interstage wiring lines 30f in the upper layer and the ground wiring line 41f in the lower layer do not electrically short circuit each other.
The insulating layers have to be formed over areas that are sufficiently larger than those of the regions in which the interstage wiring lines 30f and the ground wiring line 41f cross each other. Consequently, it is not possible to lay out the wiring lines with high density and reduce the area occupied by the wiring lines in the multilevel crossing portions 60. Therefore, providing the multilevel crossing portions 60 hinders size reduction of the longitudinally-coupled-resonator-type surface acoustic wave filter 5.